This document discusses the visitor design pattern. The visitor pattern represents an operation to be performed on elements of an object structure without changing the classes of the elements. It is useful when adding new operations to an object structure like an abstract syntax tree that contains many node types. With the visitor pattern, each node type accepts a visitor object that contains operations specific to that node type, rather than putting operations directly in each node class. This avoids changing node classes when adding new operations and separates operation code from class code for better structure and maintenance.